Monday, February 11, 2013

This extreme southern Ohio outcrop exposes fluvial sandstone laid down during the Coal Age, Pottsville and Allegheny Groups (near transition), Pennsylvanian System, Lawrence County, Ohio. You can see this outcrop a kilometer off road in Wayne National Forest near N38 42' 59" W082 41' 11".

Case hardened rock surface exfoliating from base of promontory, note thin shadows cast on the belly of the outcrop by dark exfoliation slabs separating from lighter colored sandstone underneath. Removal of slabs may have been assisted by iron ore diggers*.

A remote promontory, Wayne National Forest, Lawrence County, Ohio. A large slump block separated along a pronounced, inclined rock joint is seen at upper right.

We found this cross-bedding sandstone promontory terminating a secondary ridge overlooking Wolcott Hollow. The promontory exhibits sedimentary structures, prominent jointing, and mass wasting by slump blocks and by spalling of exfoliation slabs of case-hardened surface sandstone.

Weathering and groundwater movement has dissolved iron oxide cement beneath the rock surface and redeposited the cement in the outer centimeters of the outcrop surface, resulting in case-hardened surface layers. Loss of cement underneath the case-hardened surface has reduced the bond between the surface layer and the rock mass underneath. Addition of cement in the surface layers increases their volume creating tension between the surface layers and the underlying sandstone. The outer layer is exfoliating, exposing less weathered sandstone.

Porous and permeable sandstone bedrock often preserves detailed structures, coarse and fine, long after the sediment is lithified. Differential erosion, harder structures outlasting softer structures, can emphasize rock structures. The chemical properties of bedrock cement, in this case, iron oxides, result in the variations of hardness as well as colorful appearance. Liesegang bands, alternating color bands and sometimes swirl and ring patterns, result from diffusion-reaction properties of mobile iron oxides and other cementing chemicals.

Cross-bedding interrupted by a cavity.

Liesegang banding, a colorful diffusion-reaction phenomenon common in some massive homogeneous sandstones, here, apparently migrated from origins along sedimentary laminae within a cross-bed set. Clear bands indicate top and bottom of a single bed. A mud dauber nest suggests scale.

*Lawrence County, Ohio is part of the Hanging Rock Iron District of southeast Ohio and Northern Kentucky. The clean removal of case-hardened slabs seen in the image may be the result of ore diggers using long-handled tools to break free iron rich slabs to top off a ox cart load of the Feruginous Ore gathered nearby.

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